One well known approach to obtaining quantitative spatially resolved spectral information about a scene under observation is based on forming an image of the scene on the entrance slit of a conventional line-imaging spectrograph having a real-time detector to collect the spectrum that it produces. That spectrum consists of, essentially, a series of images of the slit, each image at a particular wavelength that increases across the spectrum. The intensity variation along a single slit image corresponds to the intensity variation, at a particular wavelength, along a line in the scene. To assemble the spectral image of the entire scene, the line in the scene being imaged onto the slit is continually changed so that a spectral image of the slit is acquired for each location in the scene.
In this way, a spectral image of the entire scene is built up line by line. Scanning of the slit with respect to the scene is done, e.g., by reflecting the incoming light from a mirror that rotates about an axis parallel to the slit. An alternative scanning method, available if the image spectrometer is used on a mobile platform such as an airplane, is to orient the slit (as projected onto the ground) perpendicular to the airplane""s ground track; thus, the scanning is obtained from the movement of the platform, without auxiliary optical elements.
In some situations knowledge of the state of linear or circular polarization of the light received from the scene would be a useful accessory to the spectral information. For example, an artificial object, such as a vehicle, generally has a smoother surface than a natural background, giving rise to partial linear polarization of the light reflected from (or emitted by, in the case of infrared radiation) the object. This polarization could be exploited in detecting such an artificial object against a natural background. The valuable information about source object surface roughness and orientation contained in the polarimetric state of received radiation is particularly advantageous for passive sensing applications. However, such polarization information is not available from conventional imaging spectrographs.
The invention provides methods and apparatus for encoding information SPECTROPOLARIMETER pertaining to the polarimetric state of transversely propagated radiationxe2x80x94for example, electromagneticxe2x80x94as a modulation of the radiation""s frequency power spectrum. In accordance with the method of the invention, a beam of transverse polychromatic radiation is first resolved into two amplitude components having mutually orthogonal polarization vectors. The first component is passed to a first fast channel and the second is component to a first slow channel, thereby delaying propagation of the first and second components by respective first shorter and longer intervals. After leaving the first fast and slow channels, the delayed first and second components are linearly combined so as to form distinct first and second composite fluxes for conveyance to respective second fast and slow channels. The first composite flux contains the delayed first and second components in a first proportion; the second composite flux contains the delayed components combined at a second proportion. The second fast and slow channels delay propagation of the first and second composite fluxes by respective second shorter and longer intervals. A first difference between the first shorter and longer intervals is not equal to a second difference between the second shorter and longer intervals.
Finally, a linear weighted sum is formed from the output of the second fast and slow channels so as to form a modulated signal containing at least one interference fringe system, the features of which characterize the polarimetric state of the original radiation. The polarimetric data may be extracted by examining the fringe system as manifested in the power spectrum of the weighted sum. If a plurality of interference fringe systems is generated, one or more of the interference systems may be selected for examination separately or after combination.
If the spectral scale of the polarization-dependent modulation is narrow compared with the spectral width of the important features of the original radiation, then the modulated spectrum is not significantly degraded compared to that of the original radiation, and the invention has simultaneous spectral and polarimetric capability. Alternatively, if modulation encoding does not adequately preserve the spectrum, a complementary set of such weighted sumsxe2x80x94whose total reconstructs the original, unmodulated power spectrumxe2x80x94may be defined by appropriately choosing a plurality of weighting factor pairs, each of which defines a single weighted sum containing the desired polarimetric information; the original power spectrum may be recovered in a separate recovery operation including alternately applying the other weighting factors in the complementary set and summing the plurality of resulting fringe systems.
In a preferred embodiment, appropriate for examining light, herein synonymous with electromagnetic radiation, especially in the visible or infrared range, birefringent media are used to perform the resolving and delaying functions. By definition a birefringent medium has differing values of refractive index, and thus differing values of propagation velocity through the medium, for various electric field orientations of propagating radiation. In such an embodiment, light originating from the scene of interest traveling along a propagation axis is passed to a first birefringent material having mutually perpendicular fast and slow axes both substantially perpendicular to the propagation axis. The first birefringent medium resolves the original light into a first component having a polarization vector coincident with the fast axis and a second component having a polarization vector coincident with the slow axis. The amplitudes of the first and second components are the projections of the original, arbitrarily oriented electric field vector onto the fast and slow axes. Over the components"" course through the medium, propagation of the first component is delayed by a first shorter interval and that of the second component by a first longer interval.
The first and second components leave the first birefringent medium and enter a second birefringent medium having mutually perpendicular fast and slow axes both substantially perpendicular to the propagation axis and rotated relative to the first medium""s axes so as to form an acute angle therewith. The second birefringent medium in turn resolves each of the delayed first and second components into two subcomponents. One subcomponent produced from each of the delayed first and second components has polarization vector coincident with the second medium""s fast axis; the other such subcomponent has polarization vector coincident with the corresponding slow axis. As the four subcomponents traverse the second medium, the two subcomponents whose amplitudes are projections onto the second fast axis of the respective electric field amplitudes of the first and second components are thus delayed by a second shorter interval, which is shorter than a second longer interval, by which the two subcomponents which are similarly projections onto the second slow axis are delayed. A first difference between the first longer and shorter intervals is not equal to a second difference between the second longer and shorter intervals.
The four delayed subcomponents exiting the second medium comprise two complementary interference fringe systems in the power spectrum domain having mutually orthogonal polarizations. A polarizer receiving the four delayed subcomponents applies weighting factors to the subcomponents based on the orientation of the polarizer transmission axis with respect to the second fast and slow axes. The polarizer selects one of the fringe systems, which appear as channeled spectra in the power spectrum of the polarizer output, and suppresses the other. Analysis of the frequency dependence of the power spectrum of the radiation leaving the polarizer renders information about the polarization state of the original radiation, including the degree of polarization, whether the radiation was polarized elliptically or linearly, and the polarization orientation. Rotation of the polarizer transmission axis by 90xc2x0 selects the complementary fringe system. Orienting the transmission axis alternately between some angle and its 90xc2x0 complement and summing the respective intensity spectra recovers the unmodulated spectrum of the original radiation, if required. Or, the transmission axis of the polarizer remains stationary and a 90xc2x0-twist nematic liquid crystal cell used to determine the portion of the delayed subcomponents received by the polarizer.
The invention is especially advantageous when used in conjunction with a conventional imaging spectrograph and detector, such as are familiar in the prior art. The elongated slit of such a spectrograph enables simultaneous spectral analysis of a column of beams, thereby imparting the ability to collect data describing the variation of polarimetric and spectral properties simultaneously along one spatial dimensional. An element introducing relative motion with respect to the scene of interest, such as a moving sensor platform or a scanning mirror broadens the capability of the invention to include time-sequential two-dimensional imaging.